Ultraviolet Radiation and the Photobiology of Earth's Early Oceans

During the Archean era (3.9–2.5 Ga ago) the earth was dominatedby an oceanic lithosphere. Thus, understanding how life aroseand persisted in the Archean oceans constitutes a majorchallenge in understanding early life on earth. Using aradiative transfer model of the late Archean oceans, thephotobiological environment of the photic zone and the surfacemicrolayer is explored at the time before the formation of a significant ozonecolumn. DNA damage rates might have been approximately threeorders of magnitude higher in the surface layer of the Archeanoceans than on the present-day oceans, but at 30 m depth,damage may have been similar to the surface of the present-dayoceans. However at this depth the risk of being transported to surface waters in the mixed layer was high. The mixed layer mayhave been inhabited by a low diversity UV-resistant biota. Butit could have been numerically abundant. Repair capabilitiessimilar to Deinococcus radiodurans would be sufficient tosurvive in the mixed layer. Diversity may have beengreater in the region below the mixed layer and above the lightcompensation point corresponding to today's `deep chlorophyllmaximum'. During much of the Archean the air-water interface wasprobably an uninhabitable extreme environment for neuston. Thehabitability of some regions of the photic zone is consistentwith the evidence embodied in the geologic record, whichsuggests an oxygenated upper layer in the Archean oceans. Duringthe early Proterozoic, as ozone concentrations increased to acolumn abundance above 1 × 10¹⁷ cm^-2, UV stresswould have been reduced and possibly a greater diversity oforganisms could have inhabited the mixed layer. However,nutrient upwelling from newly emergent continental crusts mayhave been more significant in increasing total planktonic abundance inthe open oceans and coastal regions than photobiologicalfactors. The phohobiological environment of the Archean oceanshas implications for the potential cross-transfer of life betweenother water bodies of the early Solar System, possibly on earlyMars or the water bodies of a wet, early Venus.     

Electron Paramagnetic Resonance Study of a Photosynthetic Microbial Mat and Comparison with Archean Cherts

Organic radicals in artificially carbonized biomass dominated by oxygenic and non-oxygenic photosynthetic bacteria, Microcoleus chthonoplastes-like and Chloroflexus-like bacteria respectively, were studied by Electron Paramagnetic Resonance (EPR) spectroscopy. The two bacteria species were sampled in mats from a hypersaline lake. They underwent accelerated ageing by cumulative thermal treatments to induce progressive carbonization of the biological material, mimicking the natural maturation of carbonaceous material of Archean age. For thermal treatments at temperatures higher than 620 °C, a drastic increase in the EPR linewidth is observed in the carbonaceous matter from oxygenic photosynthetic bacteria and not anoxygenic photosynthetic bacteria. This selective EPR linewidth broadening reflects the presence of a catalytic element inducing formation of radical aggregates, without affecting the molecular structure or the microstructure of the organic matter, as shown by Raman spectroscopy and Transmission Electron Microscopy. For comparison, we carried out an EPR study of organic radicals in silicified carbonaceous rocks (cherts) from various localities, of different ages (0.42 to 3.5 Gyr) and having undergone various degrees of metamorphism, i.e. various degrees of natural carbonization. EPR linewidth dispersion for the most primitive samples was quite significant, pointing to a selective dipolar broadening similar to that observed for carbonized bacteria. This surprising result merits further evaluation in the light of its potential use as a marker of past bacterial metabolisms, in particular oxygenic photosynthesis, in Archean cherts.     

Early Archean origin of Photosystem II

Photosystem II is a photochemical reaction center that catalyzes the light‐driven oxidation of water to molecular oxygen. Water oxidation is the distinctive photochemical reaction that permitted the evolution of oxygenic photosynthesis and the eventual rise of eukaryotes. At what point during the history of life an ancestral photosystem evolved the capacity to oxidize water still remains unknown. Here, we study the evolution of the core reaction center proteins of Photosystem II using sequence and structural comparisons in combination with Bayesian relaxed molecular clocks. Our results indicate that a homodimeric photosystem with sufficient oxidizing power to split water had already appeared in the early Archean about a billion years before the most recent common ancestor of all described Cyanobacteria capable of oxygenic photosynthesis, and well before the diversification of some of the known groups of anoxygenic photosynthetic bacteria. Based on a structural and functional rationale, we hypothesize that this early Archean photosystem was capable of water oxidation to oxygen and had already evolved protection mechanisms against the formation of reactive oxygen species. This would place primordial forms of oxygenic photosynthesis at a very early stage in the evolutionary history of life.     

A likely role for anoxygenic photosynthetic microbes in the formation of ancient stromatolites

Although cyanobacteria are the dominant primary producers in modern stromatolites and other microbialites, the oldest stromatolites pre-date geochemical evidence for oxygenic photosynthesis and cyanobacteria in the rock record. As a step towards the development of laboratory models of stromatolite growth, we tested the potential of a metabolically ancient anoxygenic photosynthetic bacterium to build stromatolites. This organism, Rhodopseudomonas palustris, stimulates the precipitation of calcite in solutions already highly saturated with respect to calcium carbonate, and greatly facilitates the incorporation of carbonate grains into proto-lamina (i.e. crusts). The appreciable stimulation of the growth of proto-lamina by a nonfilamentous anoxygenic microbe suggests that similar microbes may have played a greater role in the formation of Archean stromatolites than previously assumed.     

Simplicity in complexity: the photosynthetic reaction center performs as a simple 0.2 V battery.

The photosynthetic reaction center is one of the most complicated molecular complexes. Transducing photon energy to a transmembrane electrochemical potential difference for protons, it is the direct or indirect energy source for virtually all life. We show here that it operates in a simple, battery-like manner, with a maximum potential of 0.20 V. Intriguingly this is only one fifth of the energy of the absorbed photon.     

Archean Iron-Based Metabolism Analysis and the Photoferrotrophy-Driven Hypothesis of Microbial Magnetotaxis Origin

Despite its biological and geological significance, the origin of microbial magnetosome biomineralization, as well as the evolution of magnetotaxis, is still not well understood. Recently, the origin of magnetotaxis has been proposed to already exist in the Archean Eon. However, the Archean environment was fully anoxic. Therefore, what was the reason for the evolution of magnetotaxis in the anoxic Archean ocean and what mechanism could lead to the formation of single domain-sized magnetite nanoparticles that are a necessary condition of magnetotaxis functionality? Since the genetically controlled magnetosomes formation is extremely energetically demanding, in this review, we analyze Archean anoxic iron-based metabolism and we delineate the alternative possibilities of non-genetically controlled magnetosomes precursor origin as a necessary condition of magnetotaxis emergence. We show that coupling of anoxygenic photosynthesis with ferrous iron as an electron donor, with anaerobic respiration with ferric iron as an electron acceptor, provided sufficient material for non-genetically controlled magnetite formation. The co-evolution of cyanobacteria is suggested as the possible environmental pressure responsible for the emergence of Archean magnetotaxis. In accordance with the hypothesis of the reactive oxygen species-protective function of the first magnetosomes, we show that the formation of single domain-sized magnetite nanoparticles did not have to be initially connected with magnetotaxis origin, neither had to be genetically controlled nor intracellular. Instead, it could result from the long-lasting ambient pressure of metabolically produced extracellular iron oxide minerals in photoferrotrophs together with the emergence of local oxygen oases. The presence of oxygen could favor cells with the ability to navigate into oxic-anoxic transition zones since the oxygen was entirely toxic to Archean life. This evolutionary advantageous trait could finally result in a niche construction origin of genes responsible for intracellular magnetosome formation, which have remained preserved until today.     

Radiation of nitrogen‐metabolizing enzymes across the tree of life tracks environmental transitions in Earth history

Nitrogen is an essential element to life and exerts a strong control on global biological productivity. The rise and spread of nitrogen‐utilizing microbial metabolisms profoundly shaped the biosphere on the early Earth. Here, we reconciled gene and species trees to identify birth and horizontal gene transfer events for key nitrogen‐cycling genes, dated with a time‐calibrated tree of life, in order to examine the timing of the proliferation of these metabolisms across the tree of life. Our results provide new insights into the evolution of the early nitrogen cycle that expand on geochemical reconstructions. We observed widespread horizontal gene transfer of molybdenum‐based nitrogenase back to the Archean, minor horizontal transfer of genes for nitrate reduction in the Archean, and an increase in the proliferation of genes metabolizing nitrite around the time of the Mesoproterozoic (~1.5 Ga). The latter coincides with recent geochemical evidence for a mid‐Proterozoic rise in oxygen levels. Geochemical evidence of biological nitrate utilization in the Archean and early Proterozoic may reflect at least some contribution of dissimilatory nitrate reduction to ammonium (DNRA) rather than pure denitrification to N₂. Our results thus help unravel the relative dominance of two metabolic pathways that are not distinguishable with current geochemical tools. Overall, our findings thus provide novel constraints for understanding the evolution of the nitrogen cycle over time and provide insights into the bioavailability of various nitrogen sources in the early Earth with possible implications for the emergence of eukaryotic life.     

Biological activity and the Earth's surface evolution: Insights from carbon,sulfur, nitrogen and iron stable isotopes in the rock record

The search for early Earth biological activity is hindered by the scarcity of the rock record. The very few exposed sedimentary rocks have all been affected by secondary processes such as metamorphism and weathering, which might have distorted morphological microfossils and biogenic minerals beyond recognition and have altered organic matter to kerogen. The search for biological activity in such rocks therefore relies entirely on chemical, molecular or isotopic indicators. A powerful tool used for this purpose is the stable isotope signature of elements related to life (C, N, S, Fe). It provides key informations not only on the metabolic pathways operating at the time of the sediment deposition, but more globally on the biogeochemical cycling of these elements and thus on the Earth's surface evolution. Here, we review the basis of stable isotope biogeochemistry for these isotopic systems. Rather than an exhaustive approach, we address some examples to illustrate how they can be used as biosignatures of early life and as proxies for its environment, while keeping in mind what their limitations are. We then focus on the covariations among these isotopic systems during the Archean time period to show that they convey important information both on the evolution of the redox state of the terrestrial surface reservoirs and on co-occurring ecosystems in the Archean.     

Decoding algal genomes: tracing back the history of photosynthetic life on Earth

The last decade has witnessed outstanding progress in sequencing the genomes of photosynthetic eukaryotes, from major cereal crops to single celled marine phytoplankton. For the algae, we now have whole genome sequences from green, red, and brown representatives, and multiple efforts based on comparative and functional genomics approaches have provided information about the unicellular origins of higher plants, and about the evolution of photosynthetic life in general. Here we present some of the highlights from such studies, including the endosymbiotic origins of photosynthetic protists and their positioning with respect to plants and animals, the evolution of multicellularity in photosynthetic lineages, the role of sex in unicellular algae, and the potential relevance of epigenetic processes in contributing to the adaptation of algae to their environment.     

Prebiotic Synthesis of Glycine from Ethanolamine in Simulated Archean Alkaline Hydrothermal Vents

Submarine hydrothermal vents are generally considered as the likely habitats for the origin and evolution of early life on Earth. In recent years, a novel hydrothermal system in Archean subseafloor has been proposed. In this model, highly alkaline and high temperature hydrothermal fluids were generated in basalt-hosted hydrothermal vents, where H₂ and CO₂ could be abundantly provided. These extreme conditions could have played an irreplaceable role in the early evolution of life. Nevertheless, sufficient information has not yet been obtained for the abiotic synthesis of amino acids, which are indispensable components of life, at high temperature and alkaline condition. This study aims to propose a new method for the synthesis of glycine in simulated Archean submarine alkaline vent systems. We investigated the formation of glycine from ethanolamine under conditions of high temperature (80–160 °C) and highly alkaline solutions (pH = 9.70). Experiments were performed in an anaerobic environment under mild pressure (0.1–8.0 MPa) at the same time. The results suggested that the formation of glycine from ethanolamine occurred rapidly and efficiently in the presence of metal powders, and was favored by high temperatures and high pressures. The experiment provides a new pathway for prebiotic glycine formation and points out the phenomenal influence of high-temperature alkaline hydrothermal vents in origin of life in the early ocean.     

The Possible Role of Volcanic Aquifers in Prebiologic Genesis of Organic Compounds and RNA

In a volcanic aquifer, a wide range of physicaland chemical conditions are not merely possible, butto be expected: relatively oxidizing and reducingenvironments both are present; hot and moderatetemperatures can be expected; distillation and refluxconditions are probable to allow concentration ofreactants, stimulation of reaction and fractionationof isotopes; apatite, hydroxides, clays and sulfideminerals are present to act as chromatographic mediafor separating compounds, to serve as catalyticsurfaces and to provide potential energy sources;supersaturated precipitation of optically activecrystals is reasonable, allowing for chromatographicseparation of racemic mixtures by the resulting fixedchiral phase; and saturated and unsaturated conditionsboth are present for promotion of constructivereactions and inhibiting destructive hydrolysisreactions. Because the multitude of physical-chemicalenvironments makes the setting robust with respect tocircumventing commonly identified problems inorigin-of-life theories, even if objections to detailsproposed herein are identified, the setting isfavorable for devising alternatives.This paper describes a theory for the genesis oforganic compounds, including RNA, in the mixing zoneof juvenile and meteoric waters above a leakysemi-confined aquifer. Starting with basic reactantsfor best-guess conditions on Archean Earth, parallelsequences of specific reactions are proposed thatculminate with RNA oligonucleotides, key molecules incontemporary life. All proposed reactions, or closeanalogues, are experimentally confirmed and all areset in plausible Archean conditions. Calculationsindicate that the proposed reactions would yield Cisotopic compositions that are consistent withobserved biologic C.     

光合细菌在水污染治理中的研究进展

光合细菌以其无毒、繁殖快、适应能力强、易人工培养等优点而在环境治理中受到重视。国内外对光合细菌的研究主要集中在水产养殖业(如净化水质,作饵料添加剂等)和生活及工业重污染水处理中的作用,关于光合细菌在景观微污染水体治理方面的作用研究较少。课题组研究发现光合细菌中的沼泽红假单胞菌对西南大学景观水中氨氮的去除率高达95%,暗示光合细菌能有效治理景观水污染。综述了光合细菌的分类、脱氮除磷原理以及目前光合细菌在治理有机废水、重金属废水和养殖污水方面的应用,并展望了光合细菌在处理景观微污染水体方面的应用前景,以期为进一步研究光合细菌在景观水治理中的作用提供参考。     

Archean photoautotrophy: Some alternatives and limits

From the Archean geological record, one can infer that photoautotrophy evolved early in earth history; however, the nature of this photosynthesis — whether it was predominantly bacterial or cyanobacterial — is less clearly understood. General agreement that the earth's atmosphere did not become oxygen rich before the Early Proterozoic era places constraints on theories concerning more ancient biotas. Accommodating this limitation in various ways, different workers have hypothesized (1) that blue-green algae first evolved in the Early Proterozoic; (2) that oxygen producing proto-cyanobacteria existed in the Archean but had no biochemical mechanism for coping with ambient O₂; and (3) that true cyanobacteria flourished in the Archean, but did not oxygenate the atmosphere because of high rates of oxygen consumption caused, in part, by the emanation of reduced gases from widespread Archean volcanoes.Inversion of hypothesis three leads to another, as yet unexplored, alternative. It is possible that physiologically modern blue-green algae existed in Archean times, but had low productivity. Increased rates of primary production in the Early Proterozoic era resulted in the atmospheric transition documented in strata a this age. An answer to the question of wht productivity should have changed from the Archean to the Proterozoic may lie in the differing tectonic frameworks of the two areas. The earliest evidence of widespread, stable, shallow marine platforms is found in Lower Proterozoic sedimentary sequences. In such environments, productivity was, and is, high. In contrast, Archean shallow water environments are often characterized by rapid rates of clastic and pyroclastic influx —conditions that reduced rates of benthonic primary production.This hypothesis suggests that the temporal correlation of major shifts in tectonic mode and at mospheric composition may not be fortuitous. It also suggests that sedimentary environments may have constituted a significant limit to the abundance and diversity of early life.Nothing is harder, yet nothing is more necessary, than to speak of certain things whose existence is neither demonstrable nor probable.Paper presented at the College Park Colloquia on Chemical Evolution, Limits of Life, University of Maryland, College Park, MD, October 18–20, 1978.     

Biosynthetic pathways, gene replacement and the antiquity of life

The appearance of oxygen in the Earth's atmosphere, a by‐product of oxygenic photosynthesis invented by primitive cyanobacteria, stands as one of the major events in the history of life on Earth. While independent lines of geological data suggest that oxygen first began to accumulate in the atmosphere c. 2.2 billion years ago, a growing body of biomarker data purports to push this date back fully 500 million years, based on the presumption that an oxygen‐dependent biochemistry was functional at this time. Here, we present a cautionary tale in the extension of modern biochemistry into Archean biota, identifying a suite of examples of evolutionary convergence where an enzyme catalysing a highly specific, O₂‐requiring reaction has an oxygen‐independent counterpart, able to carry out the same reaction under anoxic conditions. The anaerobic enzyme has almost certainly been replaced in many reactions by the more efficient and irreversible aerobic version that uses O₂. We suggest that the unambiguous interpretation of Archean biomarkers demands a rigorous understanding of modern biochemistry and its extensibility into ancient organisms.     

Deconstructing Earth’s oldest ichnofossil record from the Pilbara Craton,West Australia: Implications for seeking life in the Archean subseafloor

Microtextures of titanite (CaTiSiO₅) in exceptionally preserved Archean pillow lavas have been proposed as the earliest examples of microbial ichnofossils. An origin from microbial tunneling of seafloor volcanic glass that is subsequently chloritized and the tunnels infilled by titanite has been argued to record the activities of subseafloor microbes. We investigate the evidence in pillow lavas of the 3.35 Ga Euro Basalt from the Pilbara Craton, Western Australia, to evaluate the biogenicity of the microtextures. We employ a combination of light microscopy and chlorite mineral chemical analysis by EPMA (electron probe micro‐analysis) to document the environment of formation and analyze their ultrastructure using FIB‐TEM (focussed ion beam combined with transmission electron microscopy) to investigate their mode of growth. Petrographic study of the original and re‐collected material identified an expanded range of titanite morphotypes along with early anatase growth forming chains and aggregates of coalesced crystallites in a sub‐greenschist facies assemblage. High‐sensitivity mapping of FIB lamellae cut across the microtextures confirm that they are discontinuous chains of coalesced crystallites that are highly variable in cross section and contain abundant chlorite inclusions, excluding an origin from the mineralization of previously hollow microtunnels. Comparison of chlorite mineral compositions to DSDP/IODP data reveals that the Euro Basalt chlorites are similar to recent seafloor chlorites. We advance an abiotic origin for the Euro Basalt microtextures formed by spontaneous nucleation and growth of titanite and/anatase during seafloor‐hydrothermal metamorphism. Our findings reveal that the Euro Basalt microtextures are not comparable to microbial ichnofossils from the recent oceanic crust, and we question the evidence for life in these Archean lavas. The metamorphic reactions that give rise to the growth of the Euro Basalt microtextures could be commonplace in Archean pillow lavas and need to be excluded when seeking traces of life in the subseafloor on the early Earth.     

Construction and maintenance of the optimal photosynthetic systems of the leaf, herbaceous plant and tree: an eco-developmental treatise

BACKGROUND AND AIMS: The paper by Monsi and Saeki in 1953 (Japanese Journal of Botany 14: 22-52) was pioneering not only in mathematical modelling of canopy photosynthesis but also in eco-developmental studies of seasonal changes in leaf canopies. SCOPE: Construction and maintenance mechanisms of efficient photosynthetic systems at three different scaling levels--single leaves, herbaceous plants and trees--are reviewed mainly based on the nitrogen optimization theory. First, the nitrogen optimization theory with respect to the canopy and the single leaf is briefly introduced. Secondly, significance of leaf thickness in CO2 diffusion in the leaf and in leaf photosynthesis is discussed. Thirdly, mechanisms of adjustment of photosynthetic properties of the leaf within the herbaceous plant individual throughout its life are discussed. In particular, roles of sugar sensing, redox control and of cytokinin are highlighted. Finally, the development of a tree is considered. CONCLUSIONS: Various mechanisms contribute to construction and maintenance of efficient photosynthetic systems. Molecular backgrounds of these ecologically important mechanisms should be clarified. The construction mechanisms of the tree cannot be explained solely by the nitrogen optimization theory. It is proposed that the pipe model theory in its differential form could be a potential tool in future studies in this research area.     

Submarine hydrothermal vents and associated gradient environments as sites for the origin and evolution of life

Submarine hydrothermal vents are the only comtemporary geological environment which may be called truly primeval; they continue to be a major source of gases and dissolved elements to the modern ocean as they were to the Archean ocean. Then, as now, they encompassed a multiplicity of physical and chemical gradients as a direct result of interactions between extensive hydrothermal activity in the Earth's crust and the overlying oceanic and atmospheric environments. We have proposed that these gradients provided the necessary multiple pathways for the abiotic synthesis of chemical compounds, origin and evolution of precells and precell communities and, ultimately, the evolution of free-living organisms. This hypothesis is consistent with the tectonic, paleontological, and degassing history of the earth and with the use of thermal energy sources in the laboratory to synthesize amino acids and complex organic compounds. In this paper, we expand upon the geophysical, chemical, and possible microbiological analogies between contemporary and Archean hydrothermal systems and suggest several hypotheses, related to our model for the origin and evolution of life at Archean vents, which can be tested in present-day hydrothermal systems.     

Charakterisierung einer Synchronkultur von Scenedesmus obliquus,ihrer potentiellen Photosyntheseleistung und des Photosynthese-Quotienten während des Entwicklungscyclus

Summary Using synchronous cultures, the change in the potential photosynthetic capacity and the behavior of the photosynthetic quotient were investigated during the life cycle of Scenedesmus obliquus, strain D 3. Scenedesmus obliquus was synchronized under a light-dark regime of 14:10 hours. The quality of synchrony was demonstrated by complete synchronization, homogeneity, exponential growth, shortest possible life cycle and non-susceptibility of the life cycle to the synchronizing procedure. Furthermore, the synchronous culture was characterized by determination of cell number, dry weight, packed cell volume, chlorophylls a and b and the carotenoids during the life cycle.The potential photosynthetic capacity (as O₂ evolution) was measured manometrically and polarographically in white light. This capacity increased from the beginning of the light period until the 8th hour and then declined until the 16th hour, that is until just before release of daughter cell. The percentage difference between the maximum and the minimum of the photosynthetic capacity was the same in the light saturating and light limiting region of photosynthesis. — The photosynthetic quotient was measured by means of Warburg's indirect method. It proved to be constant throughout the life cycle under light saturating and light limiting conditions. The ratio of O₂ evolved to CO₂ consumed was just below unity.The results were compared with those of other authors and their significance was discussed. It was concluded that the change in the photosynthetic capacity is inherent in the normal life cycle of green algae and is not a result of the synchronizing light-dark regime.

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Erster Teil einer Habilitationsschrift der Naturwissenschaftlichen Fakultät der Philipps-Universität Marburg. Auszüge dieser Arbeit wurden auf dem International Congress of Photosynthesis Research in Freudenstadt im Juni 1968 vorgetragen.     

A morphogram for silica‐witherite biomorphs and its application to microfossil identification in the early earth rock record

Archean hydrothermal environments formed a likely site for the origin and early evolution of life. These are also the settings, however, were complex abiologic structures can form. Low‐temperature serpentinization of ultramafic crust can generate alkaline, silica‐saturated fluids in which carbonate–silica crystalline aggregates with life‐like morphologies can self‐assemble. These “biomorphs” could have adsorbed hydrocarbons from Fischer–Tropsch type synthesis processes, leading to metamorphosed structures that resemble carbonaceous microfossils. Although this abiogenic process has been extensively cited in the literature and has generated important controversy, so far only one specific biomorph type with a filamentous shape has been discussed for the interpretation of Archean microfossils. It is therefore critical to precisely determine the full distribution in morphology and size of these biomorphs, and to study the range of plausible geochemical conditions under which these microstructures can form. Here, a set of witherite‐silica biomorph synthesis experiments in silica‐saturated solutions is presented, for a range of pH values (from 9 to 11.5) and barium ion concentrations (from 0.6 to 40 mmol/L BaCl₂). Under these varying conditions, a wide range of life‐like structures is found, from fractal dendrites to complex shapes with continuous curvature. The size, spatial concentration, and morphology of the biomorphs are strongly controlled by environmental parameters, among which pH is the most important. This potentially limits the diversity of environments in which the growth of biomorphs could have occurred on Early Earth. Given the variety of the observed biomorph morphologies, our results show that the morphology of an individual microstructure is a poor criterion for biogenicity. However, biomorphs may be distinguished from actual populations of cellular microfossils by their wide, unimodal size distribution. Biomorphs grown by diffusion in silica gel can be differentiated by their continuous gradient in size, spatial density, and morphology along the direction of diffusion.     

Serpentinite and the dawn of life

Submarine hydrothermal vents above serpentinite produce chemical potential gradients of aqueous and ionic hydrogen, thus providing a very attractive venue for the origin of life. This environment was most favourable before Earth's massive CO(2) atmosphere was subducted into the mantle, which occurred tens to approximately 100 Myr after the moon-forming impact; thermophile to clement conditions persisted for several million years while atmospheric pCO(2) dropped from approximately 25 bar to below 1 bar. The ocean was weakly acid (pH ～ 6), and a large pH gradient existed for nascent life with pH 9-11 fluids venting from serpentinite on the seafloor. Total CO(2) in water was significant so the vent environment was not carbon limited. Biologically important phosphate and Fe(II) were somewhat soluble during this period, which occurred well before the earliest record of preserved surface rocks approximately 3.8 billion years ago (Ga) when photosynthetic life teemed on the Earth and the oceanic pH was the modern value of approximately 8. Serpentinite existed by 3.9 Ga, but older rocks that might retain evidence of its presence have not been found. Earth's sequesters extensive evidence of Archaean and younger subducted biological material, but has yet to be exploited for the Hadean record.